What’s going on?

The explicit method populates the array with 7 physical pointers to undef:

[0] Perl> @x = (undef)x7;;
[0] Perl> use Devel::Peek;;
[No such file or directory] Perl> Dump \@x;;
SV = RV(0x3c735d0) at 0x3c735c0
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x3c6ac88
  SV = PVAV(0x3cfafd8) at 0x3c6ac88
    REFCNT = 2
    FLAGS = (RMG)
    MAGIC = 0x3cee198
      MG_VIRTUAL = &PL_vtbl_arylen_p
      MG_TYPE = PERL_MAGIC_arylen_p(@)
      MG_FLAGS = 0x02
        REFCOUNTED
      MG_OBJ = 0x3c6a410
      SV = PVMG(0x3b1a078) at 0x3c6a410
        REFCNT = 1
        FLAGS = (GMG,SMG,pIOK)
        IV = 6
        NV = 0
        PV = 0
        MAGIC = 0x3ceea08
          MG_VIRTUAL = &PL_vtbl_arylen
          MG_TYPE = PERL_MAGIC_arylen(#)
          MG_OBJ = 0x3c6ac88
    ARRAY = 0x3cf1398
    FILL = 6
    MAX = 7
    ARYLEN = 0x3c6a410
    FLAGS = (REAL)
    Elt No. 0
    SV = NULL(0x0) at 0x3cb4b98
      REFCNT = 1
      FLAGS = ()
    Elt No. 1
    SV = NULL(0x0) at 0x3c73578
      REFCNT = 1
      FLAGS = ()
    Elt No. 2
    SV = NULL(0x0) at 0x3cb49b8
      REFCNT = 1
      FLAGS = ()
    Elt No. 3
    SV = NULL(0x0) at 0x3cb4af0
      REFCNT = 1
      FLAGS = ()

[download]

Whereas, when you pre-extend the array, no physical pointer are populated; the undef's are implied by the absence of pointers:

[0] Perl> undef @x;;
[0] Perl> $#x = 6;;
[0] Perl> Dump \@x;;
SV = RV(0x3c735d0) at 0x3c735c0
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x3c6ac88
  SV = PVAV(0x3cfafd8) at 0x3c6ac88
    REFCNT = 2
    FLAGS = (RMG)
    MAGIC = 0x3cee198
      MG_VIRTUAL = &PL_vtbl_arylen_p
      MG_TYPE = PERL_MAGIC_arylen_p(@)
      MG_FLAGS = 0x02
        REFCOUNTED
      MG_OBJ = 0x3c6a410
      SV = PVMG(0x3b1a078) at 0x3c6a410
        REFCNT = 1
        FLAGS = (GMG,SMG,pIOK)
        IV = 6
        NV = 0
        PV = 0
        MAGIC = 0x3ceea08
          MG_VIRTUAL = &PL_vtbl_arylen
          MG_TYPE = PERL_MAGIC_arylen(#)
          MG_OBJ = 0x3c6ac88
    ARRAY = 0x3cf1398
    FILL = 6
    MAX = 6
    ARYLEN = 0x3c6a410
    FLAGS = (REAL)
    Elt No. 0
    Elt No. 1
    Elt No. 2
    Elt No. 3

[download]

Perl code would treat the two the as the same, but user-written XS code makes assumptions.

Thanks for checking it.

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In reply to Re^3: What does my @x= (undef)x7; do? by BrowserUk
in thread What does my @x= (undef)x7; do? by gepebril69

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